1. Field of the Invention
This invention relates to rubber composite material as used in the formation of power transmission belts and, more particularly, to a rubber composite material with short, staple, reinforcing fibers of methaphenyleneiso-phthalamide uniformly dispersed in a rubber matrix. The invention is also directed to a method of manufacturing the composite material.
2. Background Art
Heretofore, to enhance the resistance of rubber to wear and compression, staple, reinforcing fibers have been dispersed in the rubber. It is known, for example, to uniformly distribute inorganic or organic, short, staple, reinforcing fibers in natural or synthetic matrix rubber. Commonly, the fibers have lengths of between 1 to 50 mm and make up 1 to 30 percent of the volume of the composite rubber material. The fibers are generally oriented in substantially parallel relationship and disposed with their lengths at an angle of from 20.degree.-160.degree., and preferably 90.degree., to the frictional surface of the composite material, i.e. The side surfaces of a transmission belt.
The fiber-filled rubber material has, in recent years, been particularly adaptable for use in V-ribbed belts in the automotive industry The automotive environment is particularly demanding on V-ribbed belts, particularly when such belts are used in serpentine drive systems. The engine compartments in today's automobiles are quite compact and, with the engine operating, the temperature in the compartment is elevated to high temperatures. In this environment, and particularly with high horse power engines, it is essential that the V-ribbed belts exhibit high resistance to heat, bending, tension and wear. To meet these demands, designers of prior art transmission belt systems have commonly utilized aromatic polyamide fibers to reinforce the rubber and particularly to give the rubber greater resistance to heat and more rigidity.
It is also known to improve the composite rubber material performance, and particularly the tensile strength and resistance to tearing, by adhering to the short, staple, reinforcing fibers, embedded in the matrix rubber, resorcin-formalin-rubber latex (RFL), isocyanate, and epoxy resin adhesive. While this treatment improves the performance of the rubber, the application of the adhesive by conventional techniques requires use of relatively expensive equipment and is a relatively time consuming, complicated and costly process. An additional problem is that the longevity of the belt may be dependant upon the survival of the adhesive, which time period may be shorter than desired. Further, it is difficult to uniformly adhesively treat the fibers, which may result in unpredictable belt performance, from one belt to the next.
In composite rubber material having a rubber matrix with short staple reinforcing fibers therein, it is desirable, for the optimum belt performance, to use fine fibers. That is, the fibers take up space in the rubber matrix and, if they are too large, they significantly alter the belt characterisitics. The fibers are significantly larger than other additives in the matrix material, such as carbon particles, and it is thus important that their size be controlled. In the absence of the fibers being RFL treated, to prevent fibrillating of the filaments, the filaments are generally dipped in water, cut to length, and then dried. This treatment tends to undesirably entangle and expand the fibers to have an effectively larger diameter
Another problem with the formation of prior art belts incorporating composite material with dispersed fibers in a rubber matrix is that the fibers commonly become and remain entangled with each other. During manufacture, the cut yarns, in an entangled state, are kneaded with the rubber. During the kneading process, the yarns do not untangle The tangled lumps of yarn remain intact in the end product, which detracts from the integrity of the end product. Certain parts of the rubber matrix have less fibers dispersed therein than desired, while other parts of the matrix material have clumps of tangled fibers which are not oriented as would produce the best quality belt.
One solution to this problem is to repetitively knead the composite mixture. While this eliminates some of the fiber tangling, the kneading may also undesirably alter the characteristics of the fibers and/or rubber matrix.